WEBINAR:

On-Demand

Derive from STL Map<>

Question: I created my own map class (called it Mapper) by creating a template class and deriving from STL map container class. I did so because I wanted to have some extra user-defined methods in this class. I don't have any problems compiling if I only use the methods provided by the STL map class. However, when I try to make a call to one of the new methods in Mapper, I get the following linking error:

By the way, runmapper.cpp is the driver program and mapfunc is the project name. I have three user-defined methods (one of them is SmartMatch(key)) that when I try to call, from the driver program, give me the same compilation error (linking error).

Answer: The compiler must see the definition, not just the declaration, of a template in order to instantiate it. It looks like you #included only the declaration (i.e., the prototype) of your added member function:

int Mapper::SmartMatch(std::wstring); // prototype

The compiler needs to see its definitions (implementation), too. If you're using Visual C++, my advice is to implement template member functions inline, inside the header file that declares the class template.

Note that deriving from STL containers is usually a bad idea because STL containers have no virtual destructors or virtual member functions. This can cause undefined behavior in certain conditions, for example, if you pass a pointer to std::map<> which actually points to a class derived from std::map<> and has its own destructor. In this case, the derived class's destructor isn't called:

Advertiser Disclosure:
Some of the products that appear on this site are from companies from which QuinStreet receives compensation. This compensation may impact how and where products appear on this site including, for example, the order in which they appear. QuinStreet does not include all companies or all types of products available in the marketplace.

Thanks for your registration, follow us on our social networks to keep up-to-date